Interpretive Summary: Palmer amaranth has become one of the most important weeds of agronomic crops in the southern U.S., in part, because of its proclivity to develop resistance to many commonly used herbicides. Greater understanding of ecological factors that regulate population dynamics of Palmer amaranth is needed so that growers can leverage these factors in their management strategies. Mature female Palmer amaranth plants produce prodigious amounts of seed; Palmer amaranth plants in Georgia have produced over 500,000 seed per plant. Weed management has historically focused on the prevention of seedling establishment and growth (e.g. PRE and POST herbicides, cultivation, etc.), but little attention has been provided to practices that minimize seed return and/or maximize seed depletion from the soil seedbank. One of the proposed methods of reducing the potential seedling population within the seedbank is to reposition weed seeds that are near the soil surface to positions below optimal emergence zones through soil inversion (i.e. moldboard plow). Palmer amaranth seeds from glyphosate-resistant and glyphosate-susceptible populations were buried at four depths ranging from 1 to 40 cm in for intervals ranging between 0 to 36 months, after which they were exhumed and seeds evaluated for viability. Palmer amaranth seed viability for each of the burial depths declined over time. At the initiation of the study seed viability was =96%; after 6 months of burial, viability declined to 65 to 78%. As burial depth increased, so did Palmer amaranth seed viability. By 36 months, seed viability ranged from 9% (1 cm depth) to 22% (40 cm depth). To evaluate the interest of seed herbivores to Palmer amaranth, seed traps were placed on the soil surface. Palmer amaranth seeds were removed over a three day period by fire ants and other arthropods, and rodents. However, the fate of the seed was not known. Many seed herbivores will not only consume weed seeds, but also actively disperse them. Additional studies are needed to characterize the fate of Palmer amaranth seeds once they are removed by potential herbivores from the seed traps and evaluate what factors in crop production systems can be easily altered to enhance seed herbivory.

Technical Abstract:
A greater understanding of the factors that regulate weed seed return to and persistence in the soil seedbank is needed for the management of difficult to control herbicide resistant weeds. Studies were conducted in Tifton, GA to evaluate the longevity of buried Palmer amaranth seeds and estimate the potential post-dispersal herbivory of seeds. Palmer amaranth seeds from glyphosate-resistant and glyphosate-susceptible populations were buried in nylon bags at four depths ranging from 1 to 40 cm in for intervals ranging between 0 to 36 months, after which the bags were exhumed and seeds evaluated for viability. There were no detectable differences in seed viability between Palmer amaranth populations, but there was a significant burial time by burial depth interaction. Palmer amaranth seed viability for each of the burial depths declined over time and was described by exponential decay regression models. Seed viability at the initiation of the study was =96%; after 6 months of burial, viability declined to 65 to 78%. As burial depth increased, so did Palmer amaranth seed viability. By 36 months, seed viability ranged from 9% (1 cm depth) to 22% (40 cm depth). To evaluate potential herbivory, seed traps with three levels of exclusion were constructed: 1) no-exclusion, 2) rodent exclusion, and 3) large arthropod exclusion. Each seed trap contained 100 Palmer amaranth seeds and were deployed for seven day intervals for 27 sample times. There were seasonal differences in seed recovery and differences among type of seed trap exclusion, but no interactions. Seed recovery was lower in the summer and early autumn and higher in the late winter and early spring, which may reflect the seasonal fluctuations in herbivore populations and/or the availability of other food sources. Seed recovery was greatest (44%) from the most restrictive traps, which only allowed access to small arthropods, such as fire ants. Traps that excluded rodents, but allowed access by small and large arthropods, had 34% seed recovery. In the non-exclusion traps, only 25% of seed were recovered, with evidence of rodent activity around these traps. Despite the physically small seed size, Palmer amaranth is targeted for removal from seed traps by seed herbivores, which could signify a reduction in the overall population density. In order to be successful, Palmer amaranth management programs will need to reduce soil seedbank population densities. Future studies need to address factors that enhance the depletion of the soil seedbank and evaluate how these interact with other weed control practices.